(1) Field of the Invention
The present invention relates to a mobile communication system and, more particularly, to a mobile communication system including an Access Gateway (AGW) located between a core network and an access network accommodating a plurality of base stations.
(2) Description of Related Art
In a wireless access network, a tunnel is established between a Base Station (BS) and an Access Gateway (AGW) and control messages and user data are transmitted through the tunnel, using a mobile IP (Mobile Internet Protocol) of the IETF (Internet Engineering Task Force). The tunnel of mobile IP is established by exchanging, for example, a Registration Request (RRQ) message and a Registration Reply (RRP) message of Proxy Mobile IP (PMIP) between the BS and the AGW. The formats of RRQ message and RRP message of PMIP are disclosed in IETF RFC3344, sections 3.1 and 3.2.
Meanwhile, in a wireless access network such as UMB (Ultra Mobile Broadband)/CAN (Converged Access Network) of 3rd Generation Partnership Project 2 (3GPP2), separation between a control plane for handling control messages and a user plane for handling user data is pursued. For example, 3GPP2 X. S0054-100-0 v1.0, sections 4.4 and 4.6, disclose that a data path and a signaling path are separated at an AGW.
FIG. 3 shows an example of a conventional wireless access network. A home agent (HA) 2 of mobile IP and an Authentication Authorization and Accounting (AAA) server 3 for performing user authentication, access authorization, and accounting are connected to a core network 1. Base stations (BSs) 10 (10A, 10B, . . . 10N) are connected to the core network 1 via an access gateway AGW 4. Reference numeral 7 denotes a session control apparatus (SRNC: Session Reference Network Controller) and reference numerals 20 (20A, 20B, . . . ) denote mobile stations (ATs).
The AGW 4 includes an AGW unit 5 for control use which handles control messages (control packets) and an AGW unit 6 for user data forwarding which handles user data (user packets). In the following description, the AGW unit 5 for control use is referred to as a C-AGW (Control plane AGW) and the AGW unit 6 for user data forwarding as a U-AGW (User plane AGW). In the above wireless access network, control packets are forwarded via the C-AGW 5 as indicated by dotted lines, and user packets are forwarded via the U-AGW 6 as indicated by solid lines.
FIG. 4 shows an example of a signaling sequence to be performed, for example, to establish a tunnel for forwarding user data between a BS 10A and the AGW 4 when an AT 20A is connected to the core network 1, in the wireless access network shown in FIG. 3.
When a connection request is issued from the AT 20A, an access authentication procedure is executed between the AAA server 3 and the AT 20A via the BS 10A, SRNC 7, and C-AGW 5 (SQ10a, SQ10b, SQ10c). At this time, the BS 10A is notified from the C-AGW 5 of an IP address of C-AGW 5 as AGW-ID (SQ11), and the C-AGW 5 is notified from the AT 20A of an identifier (ATID) of AT 20A to be authenticated (SQ12).
Upon completion of the access authentication of AT 20A, the BS 10A performs configurations (SQ14a, SQ14b) to establish a wireless connection between the AT 20A and the BS 10A. After that, the BS 10A transmits to the C-AGW 5 a tunnel setup request message to establish a tunnel for forwarding user data. The tunnel setup request includes the identifier (ATID) of AT 20A. In this example, a PMIP RRQ message is transmitted as the tunnel setup request (S015). In the case of a system configuration that allows the AGW 4 to establish a plurality of tunnels for the same AT, the BS 10A adds control information (“Primary”) for indicating the first tunnel setup to the PMIP RRQ message.
Upon receiving the PMIP RRQ message, the C-AGW 5 returns a reply message, which is a PMIP RRP message in this example, to the BS 10A (SQ16). The PMIP RRP message includes an IP address of U-AGW 6 as information (“Endpoint”) for indicating a termination point of the tunnel. Upon receiving the PMIP RRP message from the C-AGW 5, the BS 10A establishes a tunnel toward the U-AGW 6 specified by the “Endpoint” (SQ18). Thereby, the AT 20A transits into a state capable of communicating user data with a correspondent node connected to the core network 1 through the tunnel established between the BS 10A and the U-AGW 6 (SQ19a, SQ19b, SQ19c).
In the case where the AGW 4 includes a single U-AGW 6 as in the wireless access network shown in FIG. 3, the C-AGW 5 can return a reply message designating the same U-AGW as the Endpoint, in response to every tunnel setup request received from the base stations 10A to 10N. However, in the case where the AGW 4 comprises a C-AGW and a plurality of U-AGWs, when a tunnel setup request is received from one of base stations, the C-AGW 5 has to assign an optimum U-AGW to an AT by taking the load conditions of the U-AGWs into account. 3GPP2 X. S0054-100-0 v1.0 does not disclose about a method of assigning a specific U-AGW to each AT by the AGW 4 provided with a plurality of U-AGWs.
In a broadband mobile communication system such as UMB (Ultra Mobile Broadband), an elaborate handover control adaptable to mobile ATs is required in order to achieve high-speed data transmission with high efficiency. In the UMB communication system, BS switching control is performed so as to connect an AT to one of BSs for which both the statuses of uplink channel and downlink channel are the best, for example, by monitoring the status of uplink radio channel from the AT to each BS and the status of downlink radio channel from the BS to the AT, by the AT 20 and BSs 10.
In this case, there is a possibility that handovers of the same AT occur frequently between BSs for a short period depending on the situation of radio channels, with the result that ineffectual control procedures are executed repeatedly. If the conditions for AT handover between BSs occur frequently, it becomes difficult for BSs and AGW to follow up these handovers because a certain time is required for the tunnel setup between BS and AGW.